The present invention relates to a process for the treatment of organic wastes, and more particularly to an improved anaerobic digestion process for the treatment of organic wastes which results in the efficient recovery of high purity methane, hydrogen and oxygen.
Heretofore, organic wastes, such as excess activated sludge and human waste have been treated primarily by employing an anaerobic digestion process. Recently, city garbage is being treated by an anaerobic digestion process.
It is known that anaerobic digestion proceeds primarily by combining two reactions. One reaction is liquefaction fermentation by a facultative anaerobic bacteria (liquefying bacteria group) in which organic substances in the wastes are converted into lower molecular weight substances and decomposed volatile fatty acid. The other reaction is gasification fermentation by an obligatory anaerobic bacteria (gasifying bacteria group) in which fatty acids produced in said former reaction are converted into methane. In, in the usual mixed fermentation system, treatment is conducted for a long period of time (as long as from 20 to 50 days) in the presence and coexistence of the two above-mentioned bacteria groups in the same digester. As one method for shortening the number of days in the treatment period, a method for digesting in two separate as liquefaction fermentation and gasification fermentation steps is under testing. In the former one step process, the gas generated in the course of the fermentation consists mainly of methane and carbon dioxide containing hydrogen sulfide as a minor constituent. In the latter two step process, a mixed gas of carbon dioxide and hydrogen is produced at the liquefaction fermentation step, while in the gasification fermentation step, the same gas as in the one step process is produced, said gas containing methane and carbon dioxide and also containing a minor quantity of hydrogen sulfide. While the concentration of methane and hydrogen in these gases varies according to the feedstocks and the conditions of fermentation (condition of digestion), the concentration of methane in the methane containing gas is in the range from 45% to 70%, and as to the hydrogen containing gas it is in the range from 20% to 60%.
Conventionally, the methane gas produced by anaerobic digestion has been used not only for internal use as fuel for digestion as well as for power of agitation, but also for fuel for electric power generation. However in winter, the amount of internal consumption increases, and auxiliary fuel must also be consumed often. It is predicted that the total amount of recovered methane gas will increase in the near future due to the increase of the treating of wastes, such as sewage sludge and kitchen garbage. However, if the concentration of the methane is not enriched, the use thereof will be limited strongly as compared to high calorie gas, such as natural gas and synthetic gas from petroleum. Accordingly, it is necessary to recover the methane gas in high purity. Although, there are processes, such as a process to remove carbon dioxide under high pressure and to wash it with alkaline water, such processes are costly and are not practical. Further, the gas generated in the course of the fermentation contains hydrogen sulfide in a range between tens to hundreds ppm, and therefore, before use the hydrogen sulfide must be removed through desulfurization tower filled with desulfurization reagents and then be stored. It is known that digested sludge treated by anaerobic digestion is a good fertilizer, but when it is in the form of a slurry as it is discharged or in the form of a dehydrated cake, it has drawbacks in preservation and in transportation, and therefore, the usable area for the sludge is limited. Accordingly, manufacturing treatment such as drying and granulation, which requires new heat source, is necessary when the sludge is to be used as a fertilizer. Besides, since the concentration of BOD in the digestion treated liquid is as high as from thousands to 15 ppm, it is discharged after activated sludge treatment, and the treatment of the separated water is cumbersome. Furthermore, in the separated water, nitrogen is present in hundred ppm in the form of ammonia and phosphor is present in tens ppm in the form of phosphoric acid. Removal of such substances by activated sludge treatment is difficult. However, the water is richly nutritive.